Effect of reaction protocol on the nature and size of iron oxide nano particles obtained through solventless synthesis using iron(II)acetate: structural, magnetic and morphological studies
Magnetite and hematite nanoparticles (~ 20 nm size) have been synthesized by solventless thermal decomposition of iron(II)acetate in air under different reaction protocols. The structure of the decomposed materials is investigated by FT-IR and powder XRD, while the morphology of the particles formed was studied by SEM/TEM. The magnetic phases of the materials are quantitatively identified by 57Fe Mӧssbauer spectroscopy. EDX was used for elemental analysis. The results obtained from FT-IR, XRD and Mӧssbauer studies explicitly establish that the materials obtained are iron oxides (magnetite and hematite). The study shows a reaction time- and temperature-dependent conversion of magnetite to hematite. The underlying solid state reactions have been discussed. The present study comes across that a single organo-iron precursor can provide different pure phases of iron oxide nanoparticles depending on the reaction protocol.
KeywordsIron oxides Nanoparticles Solventless synthesis Protocol dependence Physical characterization
Among the materials ubiquitous in nature, the sixteen types of iron oxide-oxyhydroxides are important as these display a wide range of physical properties . For this variety of physical properties, iron oxides draw great interest in multiple scientific research areas for various application potentials, e.g., optoelectronics, magnetic storage, anti-corrosion, imaging, pigments, gas sensors. Among the iron oxides, magnetite Fe3O4 (Fe3+[Fe3+Fe2+]O4) is a common magnetic oxide having cubic inverse spinel structure with oxygen forming an fcc structure and Fe in the interstitial tetrahedral and octahedral sites. The magnetite nanoparticles have applications in ultrahigh density magnetic storage media, tracking, imaging, biological labeling detection and ferrofluid [2, 3, 4, 5]. On the other hand, hematite (α-Fe2O3) is a nontoxic, compatible to environment and stable magnetic iron oxide with rhombohedral structure. This material has a wide application potential in the magnetic devices, catalysis, pigments, gas sensors, photoanodes, batteries, etc. [6, 7, 8, 9, 10, 11]. Magnetic properties observed in magnetite and hematite are very interesting and are widely discussed in the literature [12, 13, 14]. In magnetite at ~ 123 K, a charge-ordering transition (Verwey transition) occurs when an ordering of Fe3+ and Fe2+ ions within the octahedral sites occurs and a drop in magnetization at the transition temperature is the signature of magnetite [13, 14]. On the other hand, at ~ 263 K, hematite exhibits a weak ferromagnetic to antiferromagnetic transition (Morin transition) owing to rearrangement of spins to antiparallel fashion along the rhombohedral (111) axis from a canted spin arrangement with respect to the basal (111) plane [15, 16, 17]. These magnetic phases of iron oxide along with subphases are very accurately detected by the application of Mӧssbauer spectroscopy [16, 18].
The magnetic and other physical properties of the iron oxide nanoparticles strongly depend on their shape, size and size-distribution, and two factors—size and surface anisotropy, control the magnetic property of these iron oxide nanoparticles [19, 20]. There are several methods for preparing iron oxide nanoparticles, e.g., solventless thermal decomposition of iron-containing compounds , electrochemical deposition , co-precipitation , ultrasound irradiation , solvothermal method , microemulsion  and hydrothermal method . Out of these methods, solventless thermal decomposition of iron-containing molecular materials is the most easy and practical technique for synthesis of iron oxide nanoparticles as this method neither requires any special instrument nor involves complicated reaction steps, and, moreover, this method provides a good control over the homogeneity, composition, purity, phase and microstructure of the decomposed products [14, 16, 20, 28]. Selection of a proper organo-iron molecular precursor with comparatively low decomposition temperature and a rational reaction protocol may result the desired magnetic iron oxide nanoparticles via solventless thermal decomposition route. The reaction mechanism involved as well as the energy required to initiate the decomposition reaction can be obtained by the reaction kinetic study [29, 30]. In continuation of our earlier attempts [14, 16] to prepare iron oxide nanoparticles, presently we report the synthesis of pure magnetite and hematite nanoparticles (size ~ 20 nm) using iron(II)acetate as the sole precursor under different reaction protocols. The materials synthesized were characterized by different physical techniques like FT-IR, powder XRD and 57Fe Mӧssbauer spectroscopy where the morphology and compositional analysis were studied by SEM/TEM and EDX, respectively. The observations made are discussed in the present paper.
The synthesized materials were characterized by FT-IR, powder XRD, SEM, EDX, TEM and Mӧssbauer spectroscopy. FT-IR study was done with a PerkinElmer made (Analytic 10.4.1) spectrometer, whereas the powder XRD study was carried out with PANalytical’s Empyrean powder diffractometer equipped with PIXcell3D detector and a Cu-Kα radiation source. Scanning electron microscope (SEM) observations were made with a JEOL JSM-6480 instrument with accelerating voltage of 20 kV equipped with the energy dispersive X-ray spectroscopy (EDX) detector from IXRF. Transmission electron microscopy (TEM) images were recorded using a JEOL JEM 3010 instrument with 300 kV accelerating voltage equipped with 2 k × 2 k Orius™ 833 SC200D Gatan CCD camera. 57Fe Mössbauer spectra were recorded using a conventional constant-acceleration spectrometer with a 57Co Mössbauer source.
3 Results and discussion
3.1 FT-IR study
3.2 Structural study
Results obtained on the analysis of the powder XRD data obtained for the decomposed materials
Value of D (nm)
The inter-planer spacing (d) values have been calculated for different (hkl) plane using the strong diffraction peaks for FA1 and FA3 which are found to vary with the crystal plane orientation. In case of FA1 for (220), (311) and (440) planes, the d values are 2.967, 2.532 and 1.485 Å, whereas for (012), (104), (110), (024) and (116) planes of FA3, the d values are 3.684, 2.700, 2.519, 1.841 and 1.697 Å. This orientation dependence of the lattice spacing indicates the presence of strain in these crystalline materials, which might be originated owing to the synthesis conditions. The value strain in these two iron oxide nanoparticles, estimated using Stokes–Wilson equation , lies in the range of 7.81 × 10−3–3.91 × 10−3 for FA1 and 7.17 × 10−3–3.30 × 10−3 for FA3. The strain generated is also related to the dislocation developed in the crystals.
3.3 Mössbauer study
Hyperfine parameters estimated from the 57Fe Mössbauer spectra recorded at room temperature for the materials obtained on thermal decomposition of iron(II)acetate under different reaction conditions
0.42 ± 0.01
0.32 ± 0.01
0.02 ± 0.01
50.4 ± 0.1
Magnetite (S1), tetrahedral
0.59 ± 0.02
0.32 ± 0.01
− 0.01 ± 0.01
48.6 ± 0.1
Magnetite (S2), octahedral
0.39 ± 0.01
0.37 ± 0.01
− 0.16 ± 0.01
51.5 ± 0.1
0.60 ± 0.08
0.32 ± 0.01
0.01 ± 0.01
49.0 ± 0.1
0.31 ± 0.01
0.38 ± 0.01
− 0.21 ± 0.01
52.2 ± 0.1
0.36 ± 0.08
0.36 ± 0.01
− 0.18 ± 0.01
51.1 ± 0.1
0.28 ± 0.01
0.37 ± 0.01
− 0.18 ± 0.01
52.2 ± 0.1
0.36 ± 0.08
0.38 ± 0.01
− 0.20 ± 0.01
51.1 ± 0.1
The Mӧssbauer spectrum for FA1 is decomposed into two subspectra S1 and S2 where the resonant line of S2 presents significant broadening in comparison to S1. While fitting, all the fit parameters were set free. Considering the estimated hyperfine field values available in the literature [18, 32], the sextets S1 and S2 with hyperfine field 50.4 T (isomer shift = 0.32 mm/s) and 48.6 T (isomer shift = 0.32 mm/s) are associated with iron in the tetrahedral and octahedral coordination sites in the magnetite structure, respectively. The population ratio of iron in the tetrahedral and octahedral sites obtained from the sub-spectral area is quite close to that of ideal ratio (1:2). However, the fitted isomer shift values obtained for the two sextets are close instead of being different as expected . These values are experimentally correct as far as the fitting procedure is adopted. Though not clear at this moment, the observed isomer shift values may be due to incomplete resolution of the experimentally observed spectrum and also related with the small particle size of FA1 sample. It is reported that the small size of magnetite particles shows temperature-dependent motional narrowing to a degree dependent on the mean particle size significantly affecting the hyperfine parameters . Further Mӧssbauer study at lower temperatures and magnetic field may provide further insights in this regard.
On the other hand, in case of FA2, the sextet S1 can be assigned to iron in magnetite, whereas the sextet S2 represents iron in hematite phase [16, 18, 41]. The population ratio of these two sites is ~ 2:3. Thus, heating of the precursor at 400 °C for ½ h results into magnetite. On further heating of FA1 at 400 °C for additional ½ h, the iron in magnetite phase at the S1 site in FA1 is partially converted to the iron in hematite in FA2, whereas the iron in magnetite phase at the S2 site in FA1 is fully converted to the iron in hematite in FA2. In FA3, both sextets represent the hematite phase of iron and the population ratio of these two sites is ~ 2:3. From this result, it is clear that on heating FA2 at 500 °C for 1 h, the iron in magnetite phase at the S1 site in FA2 is fully converted to the iron in hematite phase in FA3, whereas the iron in hematite phase at the S2 site in FA2 does not suffer any further change. In FA4, both sextets represent the hematite phase of iron and the population ratio of the two sites is ~ 3:7. Thus, it appears that on heating FA3 at 600 °C for 1 h, there is no change in the magnetic phase of the iron representing S1 and S2 sextets, but the iron in the hematite phase at the S1 site in FA3 is depoplated increasing the population of the iron in the hematite phase at the S2 site in FA4. From XRD results, the synthesized materials are of single phase except FA2 where magnetite and hematite are present in ~ 1:0.8 ratio. The Mӧssbauer study of these materials adequately supplements the results obtained from XRD studies. Thus, the Mӧssbauer spectroscopic study of the materials obtained on thermal decomposition of iron(II)acetate under various reaction conditions represents a reaction temperature and time-controlled formation of magnetite from iron(II)acetate followed by conversion of magnetite to hematite. On the other hand, while comparing the presently observed results on the Mӧssbauer and XRD studies, it is evident that Mӧssbauer linewidth increases with decreasing particle size. It is well known that magnetic characteristics of the nanomaterials are strongly affected by the particle size. When particle size becomes very small, thermal energy over the magnetic moment ordering originates the superparamagnetic relaxation phenomenon due to which broadening of the Mӧssbauer line width occurs [42, 43, 44, 45].
3.4 SEM, TEM and EDX studies
3.5 Solid state reaction
At 400 °C after ½ h: 3C4H6FeO4 + 4O2 (g) = Fe3O4 + 8CO (g) + 4CO2 (g) + 9H2 (g)
At 400 °C after 1 h: 34Fe3O4 + 3O2 (g) = 22Fe3O4 + 18α-Fe2O3
At 500 °C 1 h: 4Fe3O4 + 4α-Fe2O3 + O2 (g) = 10α-Fe2O3
Iron(II)acetate has been decomposed under four different reaction conditions by varying reaction temperature and duration of heating in air. The decomposed materials (FA1, FA2, FA3, FA4) were physically characterized by FT-IR, powder XRD, Mössbauer spectroscopy, SEM, TEM and EDX. It has been found that heating of iron(II)acetate at 400 °C for ½ h results in magnetite nanoparticles. An increase in the duration results in the partial conversion of magnetite into hematite. However, increase in the reaction temperature leads to the complete conversion of iron(II)acetate into hematite nanoparticles. Further increase in the reaction temperature leads to the significant increase in the hematite nanoparticles. Guided by the thermal decomposition profile, the possible thermal reactions are proposed supplemented by the quantitative results obtained from the powder XRD and Mössbauer studies. The study shows the reaction time- and temperature-dependent conversion of iron(II)acetate to magnetite and hematite nanoparticles.
Reaction kinetics should control the size, shape and yield of the iron oxide nanoparticles synthesized through solventless thermal decomposition process. In this regard, we are trying to explore the effect of the reaction atmosphere and heating rate as the controlling parameters of the synthesis. Thus, the present study establishes that a single organo-iron precursor can provide different pure phases of iron oxide nanoparticles depending on the reaction protocol. This technique can be employed for other metal oxide nanoparticles which are getting our attention too.
Author AD is thankful to DST-INSPIRE, Govt of India for providing a fellowship and also acknowledges UGC-DAE CSR, Indore, India, for providing financial assistance to carry out the Mössbauer spectroscopic measurements. Authors express thanks to Prof. S. Koner of Department of Chemistry, Jadavpur University, Kolkata, for providing the FT-IR data. Financial support for the thermogravimetry analyzer (STA 449 F3 Jupiter) from the Department of Science and Technology (DST), Govt. of India through a Grant (Ref. No. SR/FIST/PSI-157/2010) to the Department of Physics, Visva-Bharati, Santiniketan is gratefully acknowledged.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no competing interests.
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